tests: posix: Add CLOCK_THREAD_CPUTIME_ID unittest

Add test for 'clock_gettime(CLOCK_THREAD_CPUTIME_ID, &ts)'.

Signed-off-by: James Roy <rruuaanng@outlook.com>

Author: rruuaanng

tests/posix/timers/src/clock.c

@@ -22,11 +22,17 @@ static const struct timespec ref_ts = {1514821501, NSEC_PER_SEC / 2U};
 static const clockid_t clocks[] = {
 	CLOCK_MONOTONIC,
 	CLOCK_REALTIME,
+#ifdef CONFIG_POSIX_THREAD_CPUTIME
+	CLOCK_THREAD_CPUTIME_ID,
+#endif /* CONFIG_POSIX_THREAD_CPUTIME */
 };
 
 static const bool settable[] = {
 	false,
 	true,
+#ifdef CONFIG_POSIX_THREAD_CPUTIME
+	false,
+#endif /* CONFIG_POSIX_THREAD_CPUTIME */
 };
 
 static inline int64_t ts_to_ns(const struct timespec *ts)